ABSTRACT Difficulties with cognition are extremely common among breast cancer survivors treated with chemotherapy and can significantly impact quality of life, daily functioning, and ability to return to work. Unfortunately, there is limited evidence to guide management of cancer survivors' cognition. One promising intervention is increasing physical activity as it has been effective in improving cognition in non-cancer populations; however, few intervention trials have included cancer survivors. Breast cancer survivors often decrease physical activity following treatment and have very low levels of physical activity overall. Cognitive impairments are associated with greater anxiety and depression, while increasing physical activity has been shown to decrease anxiety and depression, suggesting that psychological variables could mediate the association between physical activity and cognitive impairment. Biological mechanisms may also link physical activity with cognition including synaptic plasticity and cellular aging. This project builds upon our previous work indicating that increased physical activity can improve objectively-measured processing speed and self-reported cognition among breast cancer survivors. The current proposal will examine whether a physical activity intervention improves cognition among 250 post-treatment breast cancer survivors (Stages I-III, <5 years post-treatment, treated with chemotherapy) who are reporting cognitive difficulties. We propose to conduct a 6-month intervention with a 12-month follow-up, 2-arm RCT comparing a physical activity intervention using individual counseling and Fitbit activity trackers (Exercise Arm), with a healthy aging attention-comparison condition (Healthy Aging Arm) to examine intervention effects on objectively measured processing speed and self- reported cognition (at 3 and 6 months) and maintenance of the effect at 12 months. The primary aim is to (1) investigate the impact of the Exercise arm on changes in cognition compared to the healthy Aging arm. The secondary aims are to (1) investigate the impact of the Exercise arm on maintenance of changes in cognition compared to the Healthy Aging arm; (2) examine candidate biological mechanisms and psychological mediators of intervention-related changes in cognition. Additionally, we will explore a dose response relationship of changes in physical activity with change in processing speed and self-reported cognition and explore the effects of the Exercise arm compared to the Healthy Aging arm on changes to other cognitive domains (memory, executive function, and attention) affected by chemotherapy. This study will contribute to the scientific, public health, and intervention literature by providing new information on the impact of physical activity for cognitive impairment in breast cancer survivors and help identify vulnerable populations and intervention targets. Findings from this study will inform guidelines for physical activity dose and intensity to improve the lives of millions of breast cancer survivors.